A 3-year-old presents with recurrent squatting episodes, central cyanosis, and a harsh ejection systolic murmur at the left upper sternal border. Echocardiography confirms Tetralogy of Fallot with a hypoplastic pulmonary valve annulus. During a hypercyanotic (Tet) spell, which mechanism is primarily responsible for the acute worsening of cyanosis?

• A Acute increase in pulmonary vascular resistance causing right-to-left shunt through ASD
• B Spasm of infundibular (subpulmonary) muscle causing dynamic RVOTO and increased right-to-left shunt through VSD ✓
• C Systemic vasodilation with decreased SVR diverting blood away from pulmonary circulation
• D Paradoxical embolism through VSD causing transient cerebral ischemia

Explanation

In Tetralogy of Fallot, hypercyanotic (Tet) spells result from spasm of the hypertrophied infundibular (subpulmonary) myocardium, causing dynamic right ventricular outflow tract obstruction. This increases right-to-left shunting across the VSD, bypassing the pulmonary circulation and delivering desaturated blood directly to the aorta. Squatting increases SVR (systemic vascular resistance) which helps reverse the shunt direction and is the physiological basis for the therapeutic maneuver. Management includes knee-chest position, morphine, IV fluids, propranolol, and phenylephrine.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.